At the physical layer of a digital communication system, a transmitter transmits data over a medium (e.g. an optical fiber or an electrical back-plane) to a receiver. In many cases the medium simply defines a single lane or channel that is used as a pipeline to carry data grouped in packets from a transmitter to a receiver. In other words, a packet originating from a transmitter travels through a single channel to a receiver so that the data symbols of the packet arrive at the receiver sequentially.
Increasing the rate at which packets can be serially transmitted through an optical fiber, as a complete unbroken sequence of data symbols, is becoming more expensive due to the extraordinary effort and rising costs of producing serial interfaces that can support increasingly higher data rate transmission and reception. Thus, the full capacity of optical fibers, in terms of the amount of data that can be passed through them, is not always realized due to cost limitations. Consequently, this slows the development of technologies and services in associated and complementary fields.
One solution has been to use multiple physical media (or lanes) to form a composite medium (channel) rather than use only a single physical medium. In such a scheme, each lane of the composite channel may be a single optical fiber or a single well-delineated frequency band within a wideband of frequencies supported by a single optical fiber as is the case in a typical DWDM architecture. Alternatively, a lane may exist within one optical fiber that is in a bundle of optical fibers, in which each optical fiber in the bundle supports a set of well-delineated frequency bands within a wideband of frequencies.
Data common to a single packet or multiple packet of a single flow can be sent in parallel across the multiple lanes of the composite medium. Beneficially, because the data common to a single packet or flow is simultaneously moving in parallel across multiple lanes, the rate at which the segmented data traverses any given lane can be significantly slower than the rate required to serially send a whole unbroken packet(s) through a single channel (lane). In this way the multiple lanes form a composite channel that provides a high data rate, the high data rate achieved through the summation of a number of lower data rates corresponding to respective lanes contained within the composite medium. Additionally, standard practice and common teachings in the art imply that each lane is treated as a separate physical layer in the communication system despite the obvious fact that each lane contributes to a single composite channel for a single point-to-point link/span.
As an example of the above, an optical fiber supporting a single 10 Gbps channel may be replaced with four 2.5 Gbps channels operated in parallel. Each 2.5 Gbps channel may be realized as either a separate optical fiber or a single frequency band (channel) within a wideband of frequencies supported in a single optical fiber. This scheme theoretically maintains the desired capacity of the connection (10 Gbps), while enabling the use of less expensive serial interfaces for each of the four 2.5 Gbps channels. It should also be understood from this example that the use of multiple lanes also allows greater granularity in selecting the combined capacity for a connection.
However, the use of multiple lanes does raise some difficulties. An obvious difficulty is that transmission and reception equipment must be provided for each lane and all such equipment must be synchronized so that the multiple lanes operate in unison to provide a composite channel capable of supporting the desired capacity.
Moreover, because each lane is treated as a separate physical layer, a packet that is to be sent across the composite medium, must be separated into a number smaller segments so that the complete packet will arrive at the receiver in a time representative of the desired high data rate. The packet segments can not be made too small because the real-rate at which meaningful data traverses the composite medium will be significantly lowered as a result of the required synchronization information that is added (typically within a header) to each segment. On the other end of the spectrum, if the segments are too large issues such as jitter, buffering and the requirement for extensive complex data equalization between the lanes will become problematic. Clearly, at either end of the spectrum on this issue, the advantage of using multiple lanes significantly diminishes.
Another related issue is how to establish a procedure to keep track of the segments so that the packets can be correctly reassembled at the receiver, without adding too much costly overhead to each segment. Previous solutions to these problems involved dividing a packet into a number of smaller fixed-sized cells. However, this leads to what is commonly known as the “65-byte problem”. Typically, the fixed-sized cells are chosen to have a payload capacity of 64 bytes (a number chosen to be compatible with a number of data processing and switching protocols). If packets are not evenly divisible by 64 (or whatever number is chosen to be the capacity of a cell's payload) then there will be a significant portion of the fixed-size cells transported through the composite channel that do not contain a high percentage of meaningful information (data). In fact, such cells may contain only small portions of meaningful information corresponding to the remainder of the packet not divisible by the size of the fixed-size cell. Thus, the available capacity of the link will not be efficiently used is and the real rate at which data traverses the composite channel will be much lower than the desired capacity that the system was hoped to provide.
Prior efforts to resolve this problem, for example Link Aggregation (LA), facilitate a system where complete packets are transmitted over one lane, employ a packet traffic director to determine which packet goes down which lane. However, to maintain packet transmission order the effectiveness of multiple lanes is now dependent on the decision criteria such as a destination and source address pair. Furthermore, to support large packet sizes this scheme induces large delays, requires unruly amounts of buffering at the receive-end and reduces the goal of achieving a high transmission rate with the use of multiple lanes. In fact, if a packet is to arrive at a receiver in a time representative of the desired high data rate, the lane on which the packet is sent would have to operate at the desired high data rate. Thus, a system based on LA would require costly serial interfaces if it is to provide a high data rate link. Another prior solution commonly known as the Link Capacity Adjustment Scheme (LCAS) employs the use of fixed synchronous envelopes. The envelopes are transmitted across the lanes periodically and the data symbols are interleaved among different envelopes. Generally, other prior solutions do not provide a flexible transport scheme because they retain the use of fixed-size data transport units (e.g. packets, cells, envelopes, etc.).